Direct steam generation solar systems with screw expanders and parabolic trough collectors: Energetic assessment at part-load operating conditions

Abstract

This paper explains a numerical optimization of a novel screw expander-based solar thermal electricity plant to evaluate the energetic benefits in specific case studies. In the proposed solar electricity generation system, which is based on the steam Rankine cycle, water is used as working fluid and storage, parabolic trough collectors as a thermal source and screw expander as power machine. Such solar system offers major advantages over conventional power plants adopting steam turbines: low operating pressures, good exploitation of low temperature heat sources, acceptable efficiency in energy conversion with steam-liquid mixtures and reduced size. Since screw expanders can operate at off-design working conditions in several situations when installed in direct steam generation solar plants, the main aim of the present paper is to establish a thermodynamic model to study the energy performance of the proposed power system when off-design operating conditions and variable solar radiation befall. To assess best operating conditions and maximum efficiency of the whole power system at part-load operating conditions, numerical optimization is then performed in a specific range of fluctuating evaporation temperatures under fixed condensation pressures.